The present invention relates to an optical disc having two or more of recording layers and a method of producing such an optical disc. Particularly, this invention relates to an optical disc having several recording layers formed in compliance with different disc specifications and a method of producing such an optical disc.
Optical discs have a large storage capacity and are available to non-contact reproduction, and hence are widely used as read-only type discs, such as CD (Compact Disc), LD (Laser Disc), and DVD-ROM (DVD-Read Only Memory), write-once type discs, such as CD-R and DVD-R, and also rewritable-type discs, such as, DVD-RAM and DVD-RW.
One type of an optical disc in widespread use at present is a DVD in compliance with the DVD-Read Only Memory specifications (referred to as the DVD-ROM specifications, hereinafter). Another type of an optical disc which will probably be popular in the near future is a next-generation optical disc that is a BLU-RAY disc (BD) in compliance with the BLU-RAY Disc Read-Only Format specifications (referred to as the BD-ROM specifications, hereinafter). It is expected that DVD and BD will coexist for a certain period.
Under such expectation, disc manufacturers have to produce both DVD and BD for each type of video content, such as movies, which is inefficient in cost performance. Moreover, users may have trouble with a choice of whether to buy a DVD of a movie or a BD player and wait for a BD of that movie.
Optical discs that consist of a DVD and a BD, such as shown in FIGS. 1 to 3 could solve such a problem.
Shown in FIG. 1 is an optical disc having a 0.6 mm-thick substrate 37 and a 0.5 mm-thick substrate 39. Formed in order on the substrate 37 are a DVD-recording layer 36 and a reflective film 44. Data to be recorded on the recording layer 36 is in compliance with the DVD-ROM specifications. Formed in order on the substrate 39 are a BD-recording layer 38, a reflective film 45, and a 0.1 mm-thick transparent layer 40. Data to be recorded on the recording layer 38 is in compliance with the BD-ROM specifications. The substrates 37 and 39 are bonded to each other via an adhesive layer 43 so that the BD-recording layer 38 faces the reflective film 44.
In reproduction, a laser beam L may be focused onto both of the DVD- and BD-recording layers 36 and 38 via the transparent layer 40 and the reflective film 45 to pick up data from both recording layers. Or, a laser beam L may be focused onto the DVD-recording layer 36 via the substrate 37 to pick up data therefrom whereas another laser beam L may be focused onto the BD-recording layer 38 via the transparent layer 40 and the reflective film 45 to pick up data therefrom.
Shown in FIGS. 2 and 3 is an optical disc having a dual-layer DVD-recording layer structure of DVD-recording layers 36 and 41, and a single BD-recording layer 38. Formed on the DVD-recording layer 36 is a reflective film 44, like shown in FIG. 1. Formed on the DVD-recording layer 41 is a semi-transparent reflective film 42. The DVD-recording layer 41 may be formed on a substrate 37, as shown in FIG. 2 or on a substrate 39 having the BD-recording layer 38 formed thereon, as shown in FIG. 3.
In FIG. 2, laser beam L is emitted via a substrate 37 and a transparent layer 40, respectively, in reproduction. In FIG. 3, a laser beam L is emitted only via a transparent layer 40 in reproduction.
As shown in FIGS. 1 to 3, several types of optical discs having both DVD- and BD-recording layers are known.
Optical discs having a dual-layer DVD-recording layer structure, such as shown in FIGS. 2 and 3, are preferable because most DVDs on the market at present are dual-layer discs having two recording layers for larger storage capacity.
However, optical discs, such as shown in FIG. 2, require laser beams to be emitted to both sides in reproduction. There is thus no space for labels on the disc surfaces, which causes difficulty in determining which is the DVD or BD surface. Thus, the optical discs become less attractive.
Therefore, ideally, the most preferable among the optical discs shown in FIGS. 1 to 3 is the one shown in FIG. 3 which has a dual-layer DVD-recording layer structure and is available to exposure to a laser beam on one side in reproduction from both DVD- and BD-recording layers.
Optical discs, such as shown in FIG. 3, however, have to be produced to meet both DVD- and BD-ROM specifications for reproduction of DVD-quality videos by DVD players and also BD-quality (hi-vision level) videos by BD players.
In reproduction from the DVD-recording layer 36, a laser beam L emitted via the transparent layer 40 must pass through a BD-layer structure of the BD-recording layer 38, the reflective film 45, and the transparent layer 40 formed on the substrate 39.
It is, however, very difficult to achieve reflectivity within both DVD- and BD-ROM specifications.
For example, a higher reflectivity for the reflective film 45 of the BD-layer structure causes a lower reflectivity on both DVD-recording layers 36 and 41, thus lower video quality in reproduction. In contrast, a lower reflectivity for the reflective film 45 for higher reproduction performance from the DVD-recording layers 36 and 41 causes difficulty in reproduction from the BD-recording layer 38.
Moreover, the size of pits formed on the BD-recording layer 38 varies depending on the thickness of the reflective film 45 in the BD-layer structure. Variation in pit size further causes variation in signal quality, such as jitter.
In other words, there is an optimum thickness Tf that gives higher signal quality and another optimum thickness for higher reflectivity within the specifications for the reflective film 45 in the BD-layer structure.
WO00/65584 teaches the thickness range from 8 to 20 nm for such a reflective film in a BD-layer structure with one beam incidence surface in reproduction. This is an optimum thickness range for the reflective film in a BD-layer structure having a single recording layer, with no DVD-layer structure. Thus, there is no discussion on reflectivity for DVD-recording layers.
As discussed above, there is one requirement for an optical disc having at least two DVD-recording layers and one BD-recording layer. It is about the reflective film formed between the BD-recording layer and the transparent layer via which a laser beam is incident in reproduction. In detail, this particular reflective film must meet both DVD- and BD-ROM specifications for its reflectivity.